nothing plays it!

hi all,got a strange thing going on.for some reason my computer has stopped being able to play any type of video file.i've tried all the media players i can find(nero,jet audio,vlc,winamp,windows media player and power dvd)every time i try to open a file with any of these i get a box saying either windows explorer has encountered a problem and needs to close or the relevant media player in use needs to close,sorry for the inconvenience.very annoying.anyone got any ideas for this???thanks,keith....